El 12-11-14 21:51, Helmut Wabnig escribió:
Hello,
may I ask the gurus a question please.

What does a SWR meter actually display?

effective power
reactive power
apparent power


What does it display in the 1 : 1 case with optimal impedance matching
between transmitter, cable and antenna, I guess it's effective power,
but what does it display when matching is off, say, 1 : 5
or even worse, when the mismatch SWR is 1 : 10 or greater?
There are two pointers in the instrument, forward and reverse,
how do they add up? Why do forward power and reverse power both
increase in their values when the antenna is strongly mismatched,
but the displayed SWR remains the same?

Please elaborate a little on the answers for a beginner like me.
Thank you,
w.


Most VSWR measuring instruments actually measure the modulus of
reflection coefficient ( |RC| ).

Voltage reflection coefficient (RC) = ratio between reflected voltage
wave and forward voltage wave.


As sinewaves have an amplitude and phase, RC has an amplitude and a
phase also. To find the power you need:

Forward power = (forward voltage)^2/Zref
reflected power = (reverse voltage)^2/Zref

Net power = Pfwd - Pref

Where Zref is mostly the transmission line impedance (for example 50 Ohms)

You can calculate RC from the reference impedance (for example 50
Ohms) and the load impedance (use complex calculus).

RC = (Zload - Zref)/(Zload + Zref).

The scale of the instrument does the VSWR calculation based on.

VSWR = (1+|RC|)/(1-|RC|).

|RC| = modulus or amplitude of reflection coefficient.

Most amateur VSWR indicators need a calibration where you need to set
the forward reading at the end of the scale. Changing the switch from
FWD to REF shows the VSWR.

Why VSWR? one can measure VSWR directly by probing the voltage along
a slotted transmission line (or slotted wave guide), and that was used
in the past. I still use it during courses on transmission lines or
lectures.

VSWR = (max amplitude along line) / (min amplitude along line).

An instrument displaying VSWR only, says nothing about the absolute
forward and reflected power, you only know the ratio.

An example: VSWR = 2.

Using the formulas this translates to |RC| = 0.3333, so the reflected
wave amplitude is 33.3% of the forward wave amplitude.

This means that the ratio between reverse and forward power is

Prev/Pfwd = 0.333^2/1^2 = 0.11 (11%)

Therefore many instuments also show power loss, for example
VSWR = 2 shows 11% also.

in case of VSWR = 1, |RC| = 0, so there is no reverse traveling wave,
hence no reverse power. The net power equals the forward power. The
amplitude along the line is constant (assuming zero attenuation).

Based on VSWR alone (not knowing the position of max voltage along the
line), you can't calculate the impedance at the position of the
instrument. For example both 25 and 100 Ohms show VSWR=2 on an
instrument with 50 Ohms reference impedance. there are infinite
impedances that have VSWR=2.

You can see reflection as a form om reactive power. In a well
terminated cable (Zref = Zload, VSWR=1), voltage and current are in
phase at al positions along the cable. In case of reflection, voltage
and current are no long in phase along the line (except for certain
positions).

If you want to transmit 50W via a 50 Ohms line, the line will carry
50V and 1A everywere along the line (both in time phase). If that
same line carries 50W net power, but the VSWR of the load is 10, then
you may find positions where the voltage is 158V and positions where
the current is 3.16A. The positions of maximum current are in between
positions of maximum voltage. At many places, current and voltage are
not in phase.

Pfwd = 151W, Pref = 101W

You can imagine that in the VSWR=10 situation the transmission line
loss is more (because of the higher voltage and current).

Change of forward power because of a change in VSWR is because of the
source and has therefore nothing to do with the instrument. A source
that has a real resistive output impedance equal to the reference
impedance should display constant forward power independent of load
impedance

Hopefully avoiding a long thread: An RF power amplifier should
deliver the stated power to a specified load, it does not say that the
output impedance equals that specified load.

In other words, an RF power amplifier generally doesn't have an
internal impedance of 50 Ohms. In that case some part of the reflected
power bounces at the amplifier-cable interface and goes towards the
VSWR instrument and interferes with the amplifier's original forward
power.



--
Wim
PA3DJS
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